Hot formed pre-coated steel part

ABSTRACT

A hot formed pre-coated steel part includes a steel sheet precoated with a metallic coating and hot formed, the coating including: 2.0 to 24.0% by weight of zinc; 1.1 to 7.0% by weight of silicon; optionally from 0.5 to 3.0% by weight of magnesium when the amount of silicon is between 1.1 and 4.0%, and optionally additional elements chosen from Pb, Ni, Zr or Hf, a content by weight of each additional element being less than 0.3% by weight, a balance being aluminum, unavoidable impurities and residual elements resulting from feeding ingots or from a passage of the steel sheet in a molten bath; and a ratio Al/Zn by weight greater than 2.9, where a microstructure in the steel after said hot forming being martensitic or martensitic-bainitic or made of at least 75% of equiaxed ferrite, 5 to 20% of martensite and bainite in an amount less than or equal to 10%.

This is a Continuation of U.S. Ser. No. 15/748,395, filed Jan. 29, 2018,which is a National Phase of International Patent ApplicationPCT/IB2016/000983, filed Jul. 11, 2016 claiming priority toInternational Patent Application PCT/IB2015/001284, filed Jul. 30, 2015.All of the above applications are hereby incorporated by referenceherein.

The present invention relates to a method for the manufacture of ahardened part starting from a steel sheet coated with a metalliccoating. The invention is particularly well suited for the manufactureof automotive vehicles.

BACKGROUND

Zinc based coatings are generally used because they allow for aprotection against corrosion thanks to barrier protection and cathodicprotection. The barrier effect is obtained by the application of ametallic coating on steel surface. Thus, the metallic coating preventsthe contact between steel and corrosive atmosphere. The barrier effectis independent from the nature of coating and substrate. On thecontrary, sacrificial cathodic protection is based on the fact that zincis a metal less noble that steel. Thus, if corrosion occurs, zinc isconsumed preferentially to steel. Cathodic protection is essential inareas where steel is directly exposed to corrosive atmosphere, like cutedges where surrounding zinc will be consumed before steel.

However, when heating steps are performed on such zinc coated steelsheets, for example press hardening or welding, cracks are observed insteel which spread from the coating. Indeed, occasionally, there is areduction of metal mechanical properties due to the presence of cracksin coated steel sheet after heating steps. These cracks appear with thefollowing conditions: high temperature; contact with a liquid metalhaving a low melting point (such as zinc) in addition to stress;heterogeneous diffusion of molten metal with substrate grain bulk andboundary. The designation for such phenomenon is liquid metalembrittlement (LME), also called liquid metal assisted cracking (LMAC).

The patent application US2013/0206824 discloses a method for producing asteel component with a metallic anti-corrosion coating from a steelsheet comprising at least 0.4% by weight Mn. The sheet steel product isannealed in a continuous furnace under an annealing atmospherecontaining up to 25% by volume H₂, 0.1% to 10% by volume NH₃, H₂O, N₂,and process-related impurities as the remainder, at a dew point between−50° C. and −5° C. at a temperature of 400 to 1100° C. for 5 to 600 s.The annealed steel sheet has a 5 to 200 μm thick nitration layer with aparticle size finer than the particle size of the inner core layer. Oncecoated with a metallic protective layer, a blank is separated from theannealed steel sheet, heated to an austenitising temperature of 780 to950° C., hot-formed, and cooled so that a hardened structure forms.

At high temperature, the nitration layer allows for a minimization ofrisk of embrittlement in a sheet steel produced even when the sheetsteel product is provided with a metallic coating. The metallic coating,which can be applied to the steel substrate, is based on Zn, Al, Zn—Al,Zn—Mg, Zn—Ni, Zn—Fe, Al—Mg, Al—Si, Zn—Al—Mg or Zn—Al—Mg—Si.

However, from an industrial point of view, because of the presence ofammoniac gas during the nitriding treatment, lines have to beredesigned. Indeed, this step has to be done in a hermetic box toprevent the leakage of this gas. This hermetic box difficult to produceresults in an increase of productivity costs. Moreover, it is difficultto find a material which can handle the corrosive ammoniac.Additionally, the nitriding treatment is added to the method forproducing the coated part. Thus, the duration of this method iselongated resulting in a loss of productivity. Finally, the nitrationlayer inhibits the wettability of zinc coatings when hot-dipgalvanization is realized.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for the manufacture ofa hardened part which does not have LME issues. An advantage of thepresent invention is, in particular, an easy to implement method inorder to obtain a part which does not have LME issues generated by thehot-forming.

The present invention provides a method for the manufacture of ahardened part comprising the following steps:

-   -   A) the provision of a steel sheet pre-coated with a metallic        coating comprising from 2.0 to 24.0% by weight of zinc, from 1.1        to 7.0% by weight of silicon, optionally from 1.1 to 8.0% by        weight of magnesium when the amount of silicon is between 1.1        and 4.0%, and optionally additional elements chosen from Pb, Ni,        Zr, or Hf, the content by weight of each additional element        being less than 0.3% by weight, the balance being aluminum and        unavoidable impurities and residuals elements from feeding        ingots or from the passage of the steel sheet in the molten        bath, wherein the ratio Al/Zn is above 2.9,    -   B) the cutting of the coated steel sheet to obtain a blank;    -   C) the thermal treatment of the blank at a temperature between        840 and 950° C. to obtain a fully austenitic microstructure in        the steel,    -   D) the transfer of the blank into a press tool,    -   E) the hot-forming of the blank to obtain a part,    -   F) the cooling of the part obtained at step E) in order to        obtain a microstructure in steel being martensitic or        martensito-bainitic or made of at least 75% of equiaxed ferrite,        from 5 to 20% of martensite and bainite in amount less than or        equal to 10%.

The present invention also provides a part formed from the method above.The partmay advantageously include a microstructure of the metalliccoating comprising an interdiffusion layer Fe+Fe₃Al, AlFe intermetallicphases containing dissolved Si and Zn and binary Zn—Al and Si-richphases.

The present invention further provides use of the parts for themanufacture of an automotive vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

To illustrate the invention, various embodiments and trials ofnon-limiting examples will be described, particularly with reference tothe following FIGURE:

FIG. 1 illustrates a schematic representation of the compositionaccording to the invention comprising from 2.0 to 24.0% of zinc, from1.1 to 7.0% by weight of silicon and optionally magnesium when theamount of silicon is between 1.1 and 4.0%.

DETAILED DESCRIPTION

The designation “steel” or “steel sheet” means a steel sheet for presshardening process having a composition allowing the part to achieve ahigher tensile strength above or equal to 500 MPa, preferably above orequal to 1000 MPa, advantageously above or equal to 1500 MPa. The weightcomposition of steel sheet is preferably as follows: 0.03%≤C≤0.50%;0.3%≤Mn≤3.0%; 0.05%≤Si≤0.8%; 0.015%≤Ti≤0.2%; 0.005%≤Al≤0.1%;0%≤Cr≤2.50%; 0%≤S≤0.05%; 0%≤P≤0.1%; 0%≤B≤0.010%; 0%≤Ni≤2.5%; 0%≤Mo≤0.7%;0%≤Nb≤0.15%; 0%≤N≤0.015%; 0%≤Cu≤0.15%; 0%≤Ca≤0.01%; 0%≤W≤0.35%, thebalance being iron and unavoidable impurities from the manufacture ofsteel.

For example, the steel sheet is 22MnB5 with the following composition:0.20%≤C≤0.25%; 0.15%≤Si≤0.35%; 1.10%≤Mn≤1.40%; 0%≤Cr≤0.30%; 0%≤Mo≤0.35%;0%≤P≤0.025%; 0%≤S≤0.005%; 0.020%≤Ti≤0.060%; 0.020%≤Al≤0.060%;0.002%≤B≤0.004%, the balance being iron and unavoidable impurities fromthe manufacture of steel.

The steel sheet can be Usibor®2000 with the following composition:0.24%≤C≤0.38%; 0.40%≤Mn≤3%; 0.10%≤Si≤0.70%; 0.015%≤Al≤0.070%; 0%≤Cr≤2%;0.25%≤Ni≤2%; 0.020%≤Ti≤0.10%; 0%≤Nb≤0.060%; 0.0005%≤B≤0.0040%;0.003%≤N≤0.010%; 0.0001%≤S≤0.005%; 0.0001%≤P≤0.025%; it being understoodthat the contents of titanium and nitrogen satisfy Ti/N>3.42; and thatthe contents of carbon, manganese, chromium and silicon satisfy:

${{2.6C} + \frac{Mn}{5.3} + \frac{Cr}{13} + \frac{Si}{15}} \geq {1.1\%}$

the composition optionally comprising one or more of the following:0.05% Mo 0.65%; 0.001% W 0.30%; 0.0005% Ca 0.005%, the balance beingiron and unavoidable impurities from the manufacture of steel.

For example, the steel sheet is Ductibor®500 with the followingcomposition: 0.040%≤C≤0.100%; 0.80%≤Mn≤2.00%; 0%≤Si≤0.30%; 0%≤S≤0.005%;0%≤P≤0.030%; 0.010%≤Al≤0.070%; 0.015%≤Nb≤0.100%; 0.030%≤Ti≤0.080%;0%≤N≤0.009%; 0%≤Cu≤0.100%; 0%≤Ni≤0.100%; 0%≤Cr≤0.100%; 0%≤Mo≤0.100%;0%≤Ca≤0.006%, the balance being iron and unavoidable impurities from themanufacture of steel.

Steel sheet can be obtained by hot rolling and optionally cold rollingdepending on the desired thickness, which can be for example between 0.7and 3.0 mm.

The invention relates to a method for the manufacture of a hardened partwhich does not have LME issue. Firstly, this method comprises theprovision of steel sheet pre-coated with a metallic coating comprisingfrom 2.0 to 24.0% by weight of zinc, from 1.1 to 7.0% by weight ofsilicon, optionally from 1.1 to 8.0% by weight of magnesium when theamount of silicon is between 1.1 and 4.0%, and optionally additionalelements chosen from Pb, Ni, Zr, or Hf, the content by weight of eachadditional element being less than 0.3% by weight, the balance beingaluminum and unavoidable impurities and residuals elements, wherein theratio Al/Zn is above 2.9.

The coating composition is illustrated FIG. 1. According to theinvention, the coating comprises optionally from 1.1 to 8.0% by weightof magnesium when the amount of silicon is between 1.1 and 4.0%. Theoptional presence of magnesium is represented in grey in this FIGURE.

Without willing to be bound by any theory, it seems that if theseconditions are not met, LME issue appears because Zn-rich phases are intoo high amount and liquid zinc can diffuse towards the steel/coatinginterface and create macro-cracks in the steel.

Preferably, the metallic coating does not comprise elements selectedamong Cr, Mn, Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb or theircombinations. In another preferred embodiment, the metallic coating doesnot comprise any of the following compounds: Cr, Mn, Ti, Ce, La, Nd, Pr,Ca, Bi, In, Sn and Sb. Indeed, without willing to be bound by anytheory, it seems that when these compounds are present in the coating,there is a risk that the properties of the coating, such aselectrochemical potential, are altered, because of their possibleinteractions with the essential elements of the coatings.

Preferably, the ratio Al/Zn is between 5 and 9. Without willing to bebound by any theory, it has been found that when the ratio Al/Zn is notbetween 5 and 9, there is a risk that the decrease of LME issue is lessimportant because zinc is no longer in solid solution in the aluminummatrix and Zn-rich phases start to form.

Preferably, the ratio Zn/Si is between 2.9 and 8. Without willing to bebound by any theory, it has been found that when the ratio Zn/Si is notbetween 2.9 and 8, there is a risk that the decrease of LME issue isless important because the proportion of Zn-rich phases is a little toohigh in the coating

Advantageously, the coating comprises from 2.0 to 5.0%, preferably 2.1to 4.9% by weight of silicon. In another preferred embodiment, thecoating comprises from 1.5 to 3.5% by weight of silicon. In anotherpreferred embodiment, the coating comprises from 4.5 to 5.5% by weightof silicon.

Preferably, the coating comprises from 5.0 to 19.0%, preferably 5.0 to15.0%, advantageously from 10.0 to 15.0% by weight of zinc.

Advantageously, when the amount of silicon is between 1.1 and 4.0% byweight, the coating can comprise from 0.5 to 3.0%, preferably from 1.0to 2.9% by weight of magnesium. In another preferred embodiment, thecoating comprises from 3.1 to 8.0%, preferably from 4.0 to 8% by weightof magnesium.

Advantageously, the coating comprises above 71%, preferably above 76%,by weight of aluminum.

The coating can be deposited by any methods known to the man skilled inthe art, for example hot-dip galvanization process, electrogalvanizationprocess, physical vapour deposition such as jet vapor deposition orsputtering magnetron. Preferably, the coating is deposited by hot-dipgalvanization process. In this process, the steel sheet obtained byrolling is dipped in a molten metal bath.

The bath comprises zinc, silicon, aluminum and optionally magnesium. Itcan comprise additional elements chosen from Pb, Ni, Zr, or Hf, thecontent by weight of each additional element being less than 0.3% byweight. These additional elements can improve among others ductibility,coating adhesion on the steel sheet.

The bath can also contain unavoidable impurities and residuals elementsfrom feeding ingots or from the passage of the steel sheet in the moltenbath. Residual element can be iron with a content up to 3.0% by weight.

The thickness of the coating is usually between 5 and 50 μm, preferablybetween 10 and 35 μm, advantageously between 12 and 18 μm or between 26to 31 μm. The bath temperature is usually between 580 and 660° C.

After the deposition of the coating, the steel sheet is usually wipedwith nozzles ejecting gas on both sides of the coated steel sheet. Thecoated steel sheet is then cooled. Preferably, the cooling rate is aboveor equal to 15° C. s⁻¹ between the beginning of the solidification andthe end of the solidification. Advantageously, the cooling rate betweenthe beginning and the end of the solidification is superior or equal to20° C. s¹.

Then, a skin-pass can be realized and allows work hardening the coatedsteel sheet and giving it a roughness facilitating the subsequentshaping. A degreasing and a surface treatment can be applied in order toimprove for example adhesive bonding or corrosion resistance.

Then, the coated steel sheet is cut to obtain a blank. A thermaltreatment is applied to the blank in a furnace under non protectiveatmosphere at an austenitization temperature Tm usually between 840 and950° C., preferably 880 to 930° C. Advantageously, said blank ismaintained during a dwell time tm between 1 to 12 minutes, preferablybetween 3 to 9 minutes. During the thermal treatment before thehot-forming, the coating forms an alloy layer having a high resistanceto corrosion, abrasion, wear and fatigue.

After the thermal treatment, the blank is then transferred to ahot-forming tool and hot-formed at a temperature between 600 and 830° C.The hot-forming comprises the hot-stamping and the roll-forming.Preferably, the blank is hot-stamped. The part is then cooled in thehot-forming tool or after the transfer to a specific cooling tool.

The cooling rate is controlled depending on the steel composition, insuch a way that the final microstructure after the hot-forming comprisesmostly martensite, preferably contains martensite, or martensite andbainite, or is made of at least 75% of equiaxed ferrite, from 5 to 20%of martensite and bainite in amount less than or equal to 10%.

Thus, a hardened part without LME according to the invention isobtained.

Preferably, the microstructure of the coating of the part comprises aninterdiffusion layer Fe+Fe₃Al, AlFe intermetallic phases containingdissolved Si and Zn, binary Zn—Al and Si-rich phases. When magnesium ispresent in the coating, the microstructure comprises also Zn₂Mg phaseand/or Mg₂Si phase.

In a preferred embodiment, the part is a press hardened steel parthaving a variable thickness, i.e. the press hardened steel part of theinvention can have a thickness which is not uniform but which can vary.Indeed, it is possible to achieve the desired mechanical resistancelevel in the zones which are the most subjected to external stresses,and to save weight in the other zones of the press hardened part, thuscontributing to the vehicle weight reduction. In particular, the partswith non-uniform thickness can be produced by continuous flexiblerolling, i.e. by a process wherein the sheet thickness obtained afterrolling is variable in the rolling direction, in relationship with theload which has been applied through the rollers to the sheet during therolling process.

Thus, within the conditions of the invention, it is possible tomanufacture advantageously vehicle parts with varying thickness in orderto obtain for example a tailored rolled blank. Specifically, the partcan be a front rail, a seat cross member, a side sill member, a dashpanel cross member, a front floor reinforcement, a rear floor crossmember, a rear rail, a B-pillar, a door ring or a shotgun.

For automotive application, after phosphating step, the part is dippedin an e-coating bath. Usually, the thickness of the phosphate layer isbetween 1 and 2 μm and the thickness of the e-coating layer is between15 and 25 μm, preferably inferior or equal to 20 μm. The cataphoresislayer ensures an additional protection against corrosion.

After the e-coating step, other paint layers can be deposited, forexample, a primer coat of paint, a basecoat layer and a top coat layer.

Before applying the e-coating on the part, the part is previouslydegreased and phosphated so as to ensure the adhesion of thecataphoresis.

The invention will now be explained in trials carried out forinformation only. They are not limiting.

EXAMPLES

For all samples, steel sheets used are 22MnB5. The composition of thesteel is as follows: C=0.2252%; Mn=1.1735%; P=0.0126%, S=0.0009%;N=0.0037%; Si=0.2534%; Cu=0.0187%; Ni=0.0197%; Cr=0.180%; Sn=0.004%;Al=0.0371%; Nb=0.008%; Ti=0.0382%; B=0.0028%; Mo=0.0017%; As=0.0023% etV=0.0284%.

All coatings were deposited by hot-dip galvanization process.

Example 1: Cracking Resistance Test

This test is used to determine the presence of cracks obtainable afterthe hot-forming during the press hardening process.

Trials 1 to 10 were prepared and subjected to the Cracking resistancetest.

To this end, coated trials were cut in order to obtain a blank. Blankswere then heated at a temperature of 900° C. during a dwell time varyingbetween 5 and 10 minutes. Blanks were transferred into a press tool andhot-stamped in order to obtain parts having an omega shape. Then, partswere cooled to obtain a hardening by martensitic transformation.

Finally, the deformed section of parts was cut. Then, the presence ofcracks was analyzed by SEM (Scanning Electron Microscopy). 0 meansexcellent, in other words, there is no cracks at all; 1 means that thereare microcracks having a deep between 0 and 50 μm and 2 means very bad,in other words, there are macrocracks having a deep above 50 μm. Resultsare shown in the following Table 1:

thermal treatment at 900° C. Dwell Dwell Coating Thickness time = 5 time= 10 Trials Al Si Zn Mg Al/Zn Zn/Si (μm) minutes minutes 1 81 9 10 — 8.11.1 27 2 2 2 77 9 10 4 7.7 1.1 27 2 1 3 73 9 10 8 7.3 1.1 27 2 2 4 76 915 — 5.1 1.7 27 2 2 5 79 5 15 1 5.3 3.0 27 2 2 6 78 5 15 2 5.2 3.0 27 22  7* 80 5 15 — 5.3 3.0 27 0 0  8* 83 2 15 — 5.6 7.5 27 0 0  9* 86 2 102 8.6 5.0 27 0 0 10* 88 2 10 — 8.8 5.0 27 0 0 *examples according to theinvention.

All Trials according to the invention (Trials 7 to 10) show excellentbehavior during hot-stamping. Indeed, no crack appears with the partsaccording to the present invention in contrary to Trials 1 to 6.

Example 2: Tool Degradation Test

This test is used to determine the presence of scratch and coatingsmearing in the press tool after the hot-stamping.

Thus, the press tool was analyzed by naked eyes after the hot-stampingof Trials 1, 2, 4 and 7 to 10 prepared in Example 1. 0 means excellent,in other words, there is no tool degradation and no coating smearing inthe press tool; 1 means that there are scratches and slight coatingsmearing in the press tool and 2 means very bad, in other words, thereare heavy scratches and important coating smearing in the press tool.Results are shown in the following Table 2:

Covering rate after a thermal treatment at 900° C. (%) Dwell DwellCoating time = 5 time = 10 Trials Al Si Zn Mg Al/Zn Zn/Si minutesminutes 11 81 9 10 — 9.1 1.1 1 1 12 77 9 10 4 7.7 1.1 2 2 13 76 9 15 —5.1 1.7 2 1  14* 80 5 15 — 5.3 3.0 0 0  15* 83 2 15 — 5.5 7.5 0 0  16*86 2 10 2 8.6 5.0 0 0  17* 88 2 10 — 8.6 5.0 0 0 *examples according tothe invention.

Trials 14 to 17 according to the invention have an excellent behavior ina press tool in contrary to Trials 11 to 13.

What is claimed is:
 1. A hot formed pre-coated steel part, comprising asteel sheet precoated with a metallic coating and hot formed, thecoating including: 2.0 to 24.0% by weight of zinc; 1.1 to 7.0% by weightof silicon; optionally from 0.5 to 3.0% by weight of magnesium when theamount of silicon is between 1.1 and 4.0%, and optionally additionalelements chosen from Pb, Ni, Zr or Hf, a content by weight of eachadditional element being less than 0.3% by weight, a balance beingaluminum, unavoidable impurities and residual elements resulting fromfeeding ingots or from a passage of the steel sheet in a molten bath;and a ratio Al/Zn by weight greater than 2.9; a microstructure in thesteel after said hot forming being martensitic or martensitic-bainiticor made of at least 75% of equiaxed ferrite, 5 to 20% of martensite andbainite in an amount less than or equal to 10%.
 2. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises the magnesium at 0.5 to 3.0% by weight and the silicon between1.1 and 4.0% by weight.
 3. The hot formed pre-coated steel partaccording to claim 1, wherein the coating comprises at least one of theadditional elements chosen from Pb, Ni, Zr, or Hf.
 4. The hot formedpre-coated steel part according to claim 1, wherein the ratio Al/Zn isbetween 5 and
 9. 5. The hot formed pre-coated steel part according toclaim 1, wherein the coating comprises from 2.0 to 5.0% by weight ofsilicon.
 6. The hot formed pre-coated steel part according to claim 1,wherein the coating comprises from 2.1 to 4.9% by weight of silicon. 7.The hot formed pre-coated steel part according to claim 1, wherein thecoating comprises from 1.5 to 3.5% by weight of silicon.
 8. The hotformed pre-coated steel part according to claim 5, wherein the coatingcomprises from 2.0 to 3.5% by weight of silicon.
 9. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises from 5.0 to 19.0% by weight of zinc.
 10. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises from 5.0 to 15.0% by weight of zinc.
 11. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises from 5.0 to 10.0% by weight of zinc.
 12. The hot formedpre-coated steel part according to claim 5, wherein the coatingcomprises from 5.0 to 10.0% by weight of zinc.
 13. The hot formedpre-coated steel part according to claim 7, wherein the coatingcomprises from 5.0 to 10.0% by weight of zinc.
 14. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises from 0.5 to 3.0% by weight of magnesium.
 15. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises from 1.0 to 2.9% by weight of magnesium.
 16. The hot formedpre-coated steel part according to claim 12, wherein the coatingcomprises from 1.0 to 2.9% by weight of magnesium.
 17. The hot formedpre-coated steel part according to claim 13, wherein the coatingcomprises from 1.0 to 2.9% by weight of magnesium.
 18. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises greater than 71% by weight of aluminum.
 19. The hot formedpre-coated steel part according to claim 1, wherein the coatingcomprises greater than 76% by weight of aluminum.
 20. The hot formedpre-coated steel part according to claim 1, wherein a thickness of thecoating is between 5 and 50 μm.
 21. The hot formed pre-coated steel partaccording to claim 20, wherein the thickness is between 10 and 35 μm.22. The hot formed pre-coated steel part according to claim 20, whereinthe thickness is between 12 and 18 μm.
 23. The hot formed pre-coatedsteel part according to claim 20, wherein the thickness is between 26and 31 μm.
 24. The hot formed pre-coated steel part according to claim1, wherein the coating does not comprise elements selected among Cr, Mn,Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb or combinations thereof. 25.The hot formed pre-coated steel part according to claim 1, wherein thecoating does not include In and the coating does not include Sn.
 26. Thehot formed pre-coated steel part according to claim 1, wherein themicrostructure of the metallic coating comprises an interdiffusion layerFe+Fe₃Al, AlFe intermetallic phases containing dissolved Si and Zn,binary Zn—Al and Si-rich phases.
 27. The hot formed pre-coated steelpart according to claim 26, wherein the microstructure of the metalliccoating comprises at least one of a Zn₂Mg phase or a Mg₂Si phase. 28.The hot formed pre-coated steel part according to claim 1, wherein thepart is a press hardened steel part having a variable thickness.
 29. Thehot formed pre-coated steel part according to claim 28, wherein thevariable thickness is produced by a continuous flexible rolling process.30. The hot formed pre-coated steel part according to claim 1, whereinthe part is a tailored rolled blank.
 31. The hot formed pre-coated steelpart according to claim 1 wherein the part is a front rail, a seat crossmember, a side sill member, a dash panel cross member, a front floorreinforcement, a rear floor cross member, a rear rail, a B-pillar, adoor ring or a shotgun.
 32. The hot formed pre-coated steel partaccording to claim 1 wherein the steel sheet has a base made of 22MnB5steel.
 33. The hot formed pre-coated steel part according to claim 1,wherein the steel sheet has a base with a carbon content by weight ofbetween 0.24% and 0.38%.
 34. The hot formed pre-coated steel partaccording to claim 1 wherein the steel sheet has a base with acomposition by weight percentage including: 0.24%≤C≤0.38%; 0.40%≤Mn≤3%;0.10%≤Si≤0.70%; 0.015%≤Al≤0.070%; 0%≤Cr≤2%; 0.25%≤Ni≤2%;0.020%≤Ti≤0.10%; 0%≤Nb≤0.060%; 0.0005%≤B≤0.0040%; 0.003%≤N≤0.010%;0.0001%≤S≤0.005%; 0.0001%≤P≤0.025%; it being understood that thecontents of titanium and nitrogen satisfy Ti/N>3.42, and that thecontents of carbon, manganese, chromium and silicon satisfy2.6C+Mn/5.3+Cr/13+Si/151.1%, the composition optionally comprising oneor more of the following: 0.05%≤Mo≤0.65%; 0.001%≤W≤0.30%;0.0005%≤Ca≤0.005%, a balance of the steel sheet base composition beingiron and unavoidable impurities from manufacture.
 35. The hot formedpre-coated steel part according to claim 1, wherein the steel sheet hasa base with a composition by weight percentage including:0.040%≤C≤0.100%; 0.80%≤Mn≤2.00%; 0%≤Si≤0.30%; 0%≤S≤0.005%; 0%≤P≤0.030%;0.010%≤Al≤0.070%; 0.015%≤Nb≤0.100%; 0.030%≤Ti≤0.080%; 0%≤N≤0.009%; 0% Cu0.100%; 0%≤Ni≤0.100%; 0%≤Cr≤0.100%; 0% Mo 0.100%; 0% Ca 0.006%, abalance of the steel sheet base composition being iron and unavoidableimpurities from manufacture.
 36. The hot formed pre-coated steel partaccording to claim 1, wherein the steel sheet has a base with acomposition by weight percentage including: 0.03%≤C≤0.50%; 0.3%≤Mn≤3.0%;0.05%≤Si≤0.8%; 0.015%≤Ti≤0.2%; 0.005%≤Al≤0.1%; 0%≤Cr≤2.50%; 0%≤S≤0.05%;0%≤P≤0.1%; 0%≤B≤0.010%; 0%≤Ni≤2.5%; 0%≤Mo≤0.7%; 0%≤Nb≤0.15%;0%≤N≤0.015%; 0%≤Cu≤0.15%; 0%≤Ca≤0.01%; 0%≤W≤0.35%, a balance of thesteel sheet base composition being iron and unavoidable impurities fromthe manufacture of steel.
 37. The hot formed pre-coated steel partaccording to claim 1, wherein the steel sheet has a base with acomposition by weight percentage including 0.20%≤C≤0.25%;0.15%≤Si≤0.35%; 1.10%≤Mn≤1.40%; 0%≤Cr≤0.30%; 0%≤Mo≤0.35%; 0%≤P≤0.025%;0%≤S≤0.005%; 0.020%≤Ti≤0.060%; 0.020% Al 0.060%; 0.002%≤B≤0.004%, abalance of the steel sheet base composition being iron and unavoidableimpurities from the manufacture of steel.
 38. The hot formed pre-coatedsteel part according to claim 16, wherein the coating comprises greaterthan 71% by weight of aluminum.
 39. The hot formed pre-coated steel partaccording to claim 17, wherein the coating comprises greater than 71% byweight of aluminum.
 40. The hot formed pre-coated steel part accordingto claim 16, wherein the coating comprises greater than 76% by weight ofaluminum.
 41. The hot formed pre-coated steel part according to claim17, wherein the coating comprises greater than 76% by weight ofaluminum.
 42. The hot formed pre-coated steel part according to claim 1,wherein the coating after hot forming and subsequent cooling has nocracks.
 43. An automotive vehicle comprising the part according to claim1.